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Anderson Plant Genetic Resources Center CIMMYT

The importance of using cultivar resistance to control WHS has been recognized in China since the 1940s (Dai, 1941). Breeding for WHS resistance began in the late 1950s (Hsia and Hsiao et al., 1955). Until the 1970s, breeding efforts focused on selection from natural variants or irradiation-induced mutants to add WHS resistance to commercial cultivars. Most cultivars developed during this period were moderately resistant. From two susceptible to moderately susceptible Italian cultivars, Mentana and Funo, seven cultivars with improved WHS resistance and acceptable yield were selected. These cultivars, including Nanda 2419, Emai 9, Wannian 2, Yangmai 1, Yangmai 2, and Wumai 1, were released on a large scale in China (Wang and Liu, et al., 1989; Liu and Wang, 1990; Chen and Liu, et al., 1997).

From the 1970s to the early 1980s, WHS epidemics became more frequent and more severe in the middle and lower reaches of the Yangtze River Valley. To search for WHS resistance, the nationwide cooperative research network surveyed wheat germplasm collected worldwide for WHS resistance (CWSCG, 1984a). Resistant materials were identified from three sources: resistant land races, improved resistant lines, and commercial cultivars. Some of these commercial cultivars with some degree of WHS resistance, such as Yangmai 4, also have high yield potential, high combining ability, and resistance to some other important diseases. In WHS epidemic years, the yield loss due to disease in these cultivars is significantly lower than in susceptible cultivars. Hence, these cultivars are ideal agronomic parents for combining WHS resistance with other important economic traits.

Among the resistant cultivars identified during this period, Sumai 3 is the best resistance source with higher combining ability than the land races for important agronomic traits, and has been used with remarkable success in many WHS resistance breeding programs both in China and in other countries worldwide (Liu and Wang, 1990; Liu and Wang et al., 1991). Most WHS resistant materials developed in China are related to Sumai 3 (Bai and Chen et al., 1999). Sumai 3 was selected from transgressive segregation of the cross between Funo, a moderately susceptible Italian cultivar, and Taiwanxiaomai, a moderately susceptible land race from China (Bai and Chen et al., 1999). Sumai 3 has been used as a resistant parent to develop many new cultivars, such as Ning 7840, that have the same WHS resistance as Sumai 3, but carry additional genes for resistance to other diseases, such as rusts and powdery mildew, and have better agronomic characteristics than Sumai 3 (Bai and Zhou et al., 1989b). Transgressive segregation of WHS resistance is not an unusual phenomenon, and has been reported in many crosses in breeding programs in China (Liu and Wang et al., 1991). Recently, a new cross has been made between Funo and Taiwanxiaomai, and 50 lines with WHS resistance similar to Sumai 3 have been selected from the cross (Liu and Wu et al., 1996).

Since the 1980s, progress has been made in combining WHS resistance genes from resistant parents with desired economic traits from elite breeding lines to improve the degree of WHS resistance in commercial cultivars (Zhou and Xia, 1984; Yang, 1989). Cultivar Yangmai 158, which has moderate WHS resistance, has been grown on a large scale (Wang, 1997). Recently developed cultivars, such as Ningmai 7, Chuanmai 25, and Longmai 19, which have moderate WHS resistance and high yield potential, have been released for production in a large area (Gilchrist and Rajaram et al., 1997).

Recently, wheat breeders in China have been employing new strategies to combine high resistance with desired agronomic traits. These strategies include recurrent selection, alien gene transfer, and somaclonal variation (Lu and Jiang et al., 1995; Jiang and Wu, 1996; Sun and Chen et al., 1998). Here, we emphasize these three new strategies used in WHS resistance breeding programs in China (for other breeding strategies that have been used in China, see Bai, Chen, and Shaner, 1999).

Recurrent selection

To combine quantitative WHS resistance and superior agronomic characters, a modified recurrent selection method was proposed to maintain genetic diversity, to improve population WHS resistance, and to break unfavorable linkages (Wu and Shen et al., 1984). A dominant, nuclear, male-sterile gene, Ta1, was identified in Taigu County, Shanxi Province in 1980. The Ta1 gene has been transferred into commercial cultivars, such as Ningmai 3, Yangmai 4, Zhemai 1, Qianjiang 1, and Wanjian7909, which has made it possible to conduct recurrent selection in wheat breeding programs (Jiang and Wu, 1996). These five cultivars were then used as female parents to cross to 20 male parents. Some of these male parents, such as Sumai 3, Wangshuibai, and Fanshanxiaomai, had high WHS resistance, and others, such as Xiangmai 1 and Veery, had desired agronomic traits from diverse sources (Jiang and Wu, 1996). In 1985, these 100 crosses were tested in the provinces of Jiangsu, Anhui, and Zhejiang, and 75 crosses were selected to construct basic populations. Basic populations were maintained by phenotypic mass selection of elite male-sterile plants. Male-fertile plants with desired agronomic traits from the recurrent populations were selected for further cultivar development following traditional breeding procedures. By using this method, three gene pools have been developed. Long-term pools were established in the provinces of Jiangsu, Zhejiang, and Anhui. To maintain maximum genetic diversity, low selection pressure for WHS resistance was applied in these gene pools. Usually, for a long-term pool, approximately 50,000 plants were grown under natural infection conditions (Jiang and Wu, 1996). About 15 to 20% of the spikes were selected each cycle based on their agronomic performance and WHS resistance. To enhance population resistance levels and to produce new germplasm, resistance gene pools were deployed in the provinces of Jiangsu and Fujian, and the municipality of Shanghai where the weather favors head scab epidemics. In resistance gene pools, selection was made based on the number of scabbed spikelets under WHS epidemic conditions in the three locations. For each selection cycle, 5 to 10% of the plants were selected. Short-term pools were designed to improve agronomic traits so that new lines with high yield potential and acceptable WHS resistance level could be selected in a short time. Short-term pools were distributed in 10 different locations. In short-term pools, fewer than 5% of the plants were selected in each cycle based on plant height, spike traits, and WHS resistance (Jiang and Wu, 1996).

After four cycles of selection from resistance gene pools in Nanjing, Jiangsu Province, the frequency of resistant plants (less than four scabbed spikelets per spike) was increased by 4.2% per selection cycle (Jiang and Wu, 1996; Jiang and Chen et al., 1997). To date, eight cycles of selection have been completed, and elite lines TFSL037 and Changjiang 8809 have been selected. These two lines have demonstrated high resistance similar to that of Sumai 3 and high yield potential in regional yield trials (Jiang and Wu, 1996). Line W14 and several other resistant lines, which have WHS resistance similar to Sumai 3 and better agronomic traits, were selected from resistance gene pools and can be used as resistant parents in breeding programs. Lines Changjiang 9045 and Changjiang 9046 showed moderate WHS resistance and a high yield potential in four years' tests (Jiang and Wu, 1996).
From 1991 to 1995, 1,243 lines from recurrent selection programs in 12 institutes were evaluated for WHS resistance in Nanping, Fujian Province, where natural infections occur frequently and severely. Among the lines evaluated under natural conditions in the field, 229 lines were found to have high resistance to WHS (Fang and Zhang et al., 1996). Eight lines with WHS resistance and a higher yield potential than Yangmai 5 were selected (Fang and Zhu et al., 1996). By backcrossing Xianyang 84+(79)-3-1-1 and 86-5, two lines developed in Fujian Province, to the recurrent population, 12 lines with high WHS resistance were selected. Among them, five lines, including Futai 9501, Futai 9502, Futai 9503, Futai 9504, and Futai 9505, had high level of WHS resistance (Ye and Zhang et al., 1996). In another recurrent selection program, Zhe5148 and Zhe5185 selected in Zhejiang Province had much better agronomic traits than Sumai 3 and WHS resistance similar to Sumai 3 (Shen and Yu et al., 1993). Based on different breeding objectives, three sets of eight cultivars were used to establish three recurrent populations in the wheat breeding programs at South China Agricultural University (Zhang and Pan et al., 1993). After several years' selection, nine resistant lines have been developed with WHS resistance similar to Sumai 3, high yield potential, and desired agronomic traits (Zhang and Pan et al., 1993). The results indicate that recurrent selection is an effective tool to combine WHS resistance from different resources and to combine WHS resistance with desired agronomic traits.

Transferring resistance genes from alien species

To enhance genetic diversity, breeders often have turned to wild relatives of wheat as a source of WHS resistance genes. A total of 5,831 accessions of alien species were tested for head scab resistance in several institutes in China from 1977 to 1983, but no highly resistant material was found (CWSCG, 1984a; Wang and Liu et al., 1989). Recently, two hundred and seventy-six accessions from 80 species of 16 genera of Triticeae were evaluated for head scab resistance following single spikelet inoculation (Wan and Yan et al., 1997). Species of Aegilops, Crithopsis, Eremopyrum, Heteranthelium, Henrardia, Haynaldia, Teaniatherum, and annual species of Hordeum were highly susceptible to head scab. However, species of Roegneria, Elytrigia, Pseudoroegeria, Psathyrostachys, Leymus (=Elymus), Kengyilia, Agropyron, and perennial species of Hordeum were reported to have resistance to head scab. The inconsistency between the earlier studies and the later study may result from the different accessions tested. Repeated evaluation of these alien materials in different locations may provide more accurate information on their head scab resistance levels.

At Nanjing Agricultural University, fourteen wheat relative species from 11 genera were selected for evaluation of resistance to head scab by using the single floret inoculation method (Liu and Weng et al., 1989). Leymus racemosus (=Elymus giganteus), Roegneria Kamoji, and R.ciliaris had high level of resistance to head scab spread (Weng and Liu, 1989). Hybrid F₁ progenies between common wheat and three related species were successfully obtained by utilizing embryo culture (Wang and Chen et al., 1986; Weng and Liu, 1991; Weng and Wu et al., 1993). Through backcrossing these progenies with common wheat, more than 10 alien addition lines and substitution lines were identified. Integration of alien chromosomes in these lines was confirmed by mitotic and meiotic analysis combined with chromosome C-banding, in situ hybridization, isozyme analysis, and restriction fragment length polymorphism, as well as evaluation of head scab resistance. Three disomic addition lines, involving Leymus racemosus chromosome 2, 7, 14 (temporarily designed as their homologues because the genomes are not characterized yet), had WHS resistance. The resistance level was almost as high as that of Sumai 3. This result indicates that at least three chromosomes of Leymus racemosus are involved in WHS resistance (Chen and Wang et al., 1993). One common wheat -Roegneria kamoji disomic addition line and one substitution line showed resistance to WHS (Wu and Wang et al., 1997; Wang and Chen et al., 1997; Wang and Qi et al., 1999). From backcross progenies of common wheat and a wheat -Roegneria ciliaris amphiploid, three alien addition lines were identified and showed moderate level of WHS resistance (Wang and Chen et al., 1994). By combining techniques of ⁶⁰Co-gamma radiation, chromosome pairing homologous system (Ph mutant or Ph gene deletion), gametocidal genes, and tissue culture, several wheat-Leymus racemosus translocation lines have also been isolated. The wheat lines having translocations involving chromosome 2 and 7 of Leymus racemosus have high WHS resistance (Chen and Sun et al., 1998). Wheat lines with alien chromosomes were also crossed with Sumai 3 and Xiangmai 1, and several resistant lines with better agronomic traits than Sumai 3 have been selected (Chen and Liu, et al., 1997). These lines with resistance genes from Leymus racemosus are valuable breeding materials for improvement of WHS resistance.

The head scab resistance in these alien species may not surpass the resistance in wheat cultivars or breeding lines identified so far, however, using head scab resistance genes from alien sources may increase the genetic diversity of WHS resistance genes in wheat breeding programs.

Mutation breeding

Mutagenesis by treatment with gamma or UV radiation is a common practice in plant breeding programs. In WHS resistance breeding programs, progress has been made in improving WHS resistance of moderately susceptible cultivars by using irradiation treatment (Bai and Chen et al., 1999). The moderately resistant cultivar Emai 6 was selected from the moderately susceptible cultivar Nanda 2419 following treatment with ⁶⁰Co-gamma radiation (Chen and Liu et al., 1997). However, attempts to increase WHS resistance of resistant cultivars by treatment with ⁶⁰Co-gamma radiation were not successful. WHS resistance in irradiated Sumai 3 plants has been lower than in untreated plants. Irradiation treatment also reduced WHS resistance of plants derived from treated seeds of both disomic lines and monosomic lines of Sumai 3. The seeds of Sumai 3 and its 21 monosomic lines were sensitive to a medium dose (30 kGy) of ⁶⁰Co-gamma radiation. The plants generated from radiation-treated seeds also showed changes in agronomic traits, such as reduced number of tillers, shorter plant height, delay of heading stage, and a reduction in number of spikelets per spike (Zhang and Yu, 1995). The monosomic line 4A was most sensitive to ⁶⁰Co-gamma radiation treatment. Only a few irradiation treated monosomic lines had a slighly higher WHS resistance than their disomic parents, and most monosomic lines did not show a significantly higher WHS resistance than disomic Sumai 3 (Zhang and Yu, 1995). These results indicated that irradiation mutation can be used to enhance WHS resistance in cultivars with moderate or low levels of resistance, but cannot increase the resistance in highly resistant cultivars. Since most commercial cultivars have low levels of WHS resistance, irradiation breeding may be useful to improve their resistance. Using wheat tissue cultures, instead of wheat seed, may improve the efficiency of irradiation breeding (Guo and Yao et al., 1992 a, b; Li and Li et al., 1996).

Another application of irradiation breeding is to develop bridge parents to transfer head scab resistance from alien species (Chen and Sun et al., 1998). As mentioned above, several alien addition lines have been developed. Translocation between wheat chromosomes and alien chromosomes is important for development of genetically stable cultivars that have incorporated resistance genes from the alien chromosomes. Commercial cultivars were pollinated with pollen collected from alien addition line spikes that have been treated with ⁶⁰Co-gamma radiation before flowering (Chen and Sun et al., 1998). Several translocation lines were selected by this method. Wheat lines that retain most of the wheat chromosomes with integration of one small part of a chromosome from L. racemosus will be useful parents for WHS resistance breeding.

Somaclonal variation is another source for selection of resistant lines (Lu and Jiang et al., 1995). To study the somaclonal variation in WHS resistance, the F₂ and R₂ progenies from a cross between Chongyanghongmai and E'en 1 were compared (Yu, 1990). R2 plants were generated by culturing the immature embryos of the hybrid F₁ from the cross. Variation in WHS resistance was larger among R₂ progenies than among F₂ progenies, and the average WHS severity of R₂ was lower than that of F₂ (Yu, 1990). The Line 2870, selected from somaclonal variants of young embryos of F₁ plants of the cross, had WHS resistance similar to Sumai 3 (Yu, 1991). By in vitro culture of wheat embryos, line Ning 895004 was regenerated from a susceptible commercial cultivar Ningmai 3. Ning 895004, which has both higher WHS resistance and higher yield potential than Yangmai 5, has been grown in the provinces of Jiangsu, Zhejiang, and Hubei, and the municipality of Shanghai, on more than 200,000 hectares (Lu and Jiang et al., 1995; Lu, W.-Z.,1999, personal communication). Random amplified polymorphic DNA analysis showed a high level of polymorphism between Ning 895004 and its parent Ningmai 3 (Shen and Lu et al., 1996). Among 140 primers used, six produced polymorphic bands between Ning 895004 and Ningmai 3. These results provided molecular evidence of genetic changes of wheat in tissue culture.

A significant difference among wheat cultivars in sensitivity to DON produced by F. graminearum has been observed, and has been associated with WHS resistance of wheat cultivars. Thus, adding DON to media during tissue culture could select variants tolerant of DON and resistant to WHS (Wang and Chen et al., 1989; Wang and Miller, 1989; Xu and Yao et al., 1990). Scientists from several institutes in China have explored the possibility of using DON to select WHS-resistant variants (Lu and Zhou et al., 1991; Zhang and Zhang et al., 1991; Guo and Yao et al., 1992 a, b; Zhou and Cui et al., 1993; Lu and Jiang et al.,1995; Sun and Wang et al., 1995; Fu and He et al., 1996). The anther culture technique was employed to screen resistant callus in media containing F. graminearum culture filtrates. Four highly resistant plants were selected from somaclonal variation of moderately resistant cultivars of spring wheat (Guo and Yao et al., 1992). Immature embryos have been more widely used to generate somaclonal variants (Zhou and Cui et al., 1993; Sun and Wang et al., 1995; Chen and Han et al., 1995; Fu and He et al., 1996). Young embryos were cultured on media containing known levels of pure DON, or fungal culture extracts or filtrates. Plants regenerated from callus tolerant to DON were put through conventional selection breeding procedures and were tested for WHS resistance. Callus from F₁ of the cross of Line Salt- Tolerance 03 and Line No18 had a higher regeneration rate than callus from the parents, and untreated callus had a higher regeneration rate than DON-treated callus (Chen and Han et al., 1995; Fu and He et al., 1996). A number of variants have been obtained by using crude toxin as selection pressure (Zhou and Cui et al., 1993; Sun and Wang et al., 1995; Chen and Han et al., 1995; Fu and He et al., 1996). Four WHS resistant lines that had much higher resistance than their progenitors were obtained by culturing immature embryos of wheat cultivars in medium containing fungal culture extract (Sun and Wang et al., 1995). Some plants in R₁ and R₂ generations from these variants showed higher WHS resistance than the progenitor cultivars (Zhou and Cui et al., 1993). These reports showed some success in using DON to screen WHS resistant variants, however, negative effects of DON on tissue culture have also been observed. In addition to its toxicity to plant tissues, DON also had growth hormone-like activity on wheat tissues, depending on the amount of DON applied in the medium (Liu and Chen et al., 1991; Liu and Lu et al., 1993). WHS resistant somaclonal variants can also be obtained from tissue culture without adding DON to the medium (Lu and Jiang et al., 1995). Therefore, DON may not be necessary for selection of somaclonal variants for WHS resistance, and using DON to select WHS resistant variants is not recommended by some researchers (Liu and Lu et al., 1993; Liu and Chen et al., 1991).

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